Automatic gain control system



Aug. 26, 1952 R. A. EMMETT, JR

AUTOMATIC GAIN CONTROL SYSTEM Filed Dec. 18, 1945 GATE GEN I GATE GEN 2 E S L m m m m a o n n. m M A n L m T.,.m 1 m A 3 D 2 L f P L M m 9 A WT o l N v A0 a C 2 I 2 L m r w M A A R l R E 2 E 0 CT E DU D I F l F 0 mm mm M M V V RECEIVER AIN INITIATING PU SE I AGC OUT ROBERT A. EMMETT JR.

Patented Aug. 26, 1952 7 UNITED STATES PATENT "OFFICE Y 2,608,651 AUTOMATIC GAIN CONTROL SYSTEM Robert A. Emmett, J r., United S tates Navy Application December 18, 1945, Serial No. 635,823

Claims.

(Granted under the act of March 3, 1883, amended April 30. 1928; 370 0. G. 757) i This invention relates in general to automatic gain control means and, more particularly, to means associated with a radio receiver for controlling the gain of the receiver as a function of the amplitude of signal pulses of a repetitive nature received therein.

In repetitive pulse radio transmitting and receiving systems, the problem of providing automatic volume control of the receiver dependent only upon the amplitude of the received pulse signal is rendered complex due to the short duration of the signal pulses as compared with the time duration of one cycle of operation, thus providing a low average voltage level variation for wide variations in .peak pulse amplitude,-and because of the presence of many spurious pulses such as interference signals and random noise picked up by the receiver which distorts the average voltage determined from the amplitude of the signal pulse and thus yields an average value proportional to the combined amplitude of all the signals occurring during one repetitive cycle. i

- Accordingly, it is an object of this invention to provide automatic gain control means for a radio receiving system for pulse signals in which the gain. of the receiver is accuratelycontrolled as a function of the signal pulse amplitude only.

Another object of the invention is to provide automatic gain control means for a radio receivingsystem for repetitive pulse signals in which the means is rendered sensitive to the amplitude ofreceived signals substantially at the time of occurrence of the pulse signals only.

I Another object of'the invention is to provide automatic gain control means for a radio receiv ing system for repetitive pulse signals with which automatic tracking means is associated to maintain the gain control means sensitive to amplitude variations of the signal pulses only, regardless of variations in the time of occurrence of the signal pulses. i 7

Other objects, advantages and capabilities, of the invention will become apparent with the following description, taken in conjunction with the accompanying drawing, showing only a preferred embodiment of the invention, in which:

Fig. 1 is a block diagram of one embodiment of the instant automatic gain control system;

a Fig. 2 is a schematicdiagram of the automatic gain control, automatic tracking, and gate generating circuits making up the automatic gain control system.

Referring now to Fig. 1, in which is shown theblock diagram of the automatic gain control 2 system, two gates are generated, hereafter referred to as gate I and gate 2. Gate 1 is a comparatively narrow gating pulse of about 3 micro-. seconds duration. Gate 2 is about 10 microsec-r.

ends in duration. These gates are generatedflin gate generator l0 and gate generator [-1, respectively, the gate generators being triggered simultaneously by a delay circuit l2. This delay cir-- cuit I2 is a phantastron circuit (for circuit de-g tails see the circuit of which tube I02 is an 616-1 ment in Figure 2) which, when triggered by a;

cycle initiating or reference pulse recurring at the pre-established cycle repetition frequency of the pulsetransmission system associated with the receiver l3 with which the automatic gain controlsystem is adapted to operate, generates a pulse of variable width, this width being controlledby adirect current voltage. The trailing edge of this pulse is usedto trigger the gate generators 1 Illandll. i

To obtain a control voltage for this delay generator I2, the video output of the receiver I3 is passed to the plate of a gated vacuum tube voltmeter circuit l4, this circuit being gated with the output pulse of the gate generator l0. voltmeter circuit is similar to a normal shunt diode circuit except that a triode is used so that. the'tube may be turned onand off by the gate. The advantage of using this type of circuit is .that no voltage output is developed unless the signal and the gate fed thereto coincide. The

output of this tube is filtered through a direct.

current filter l5, passed through an amplifier l6, and then to a second amplifier I! known as the drift amplifier. The drift amplifier I1 is smarranged that in the absence of the voltage output from the voltmeter circuit [4 generated when the signal and gate coincide, its plate drifts in a p i tive direction. With such a voltage, the delayv circuit 12 will generate a pulse of increasing.

width, thereby timing the gate of gate generators l0 and H to occur with increasing delay with respect to the-time of occurrence of the referenceten microsecond gate will then surround the sig- Y nal in time.

Since this automatic gain control system'isf adapted to be used in a repetitive pulse trans;

This

mitting and receiving system in which the signal pulses will occur at a variable time after the occurrence of the reference pulse, means must be provided for setting the automatic tracking unit at the approximate time of occurrence of the signal pulses when the unit is set into operation.

Because the-rateiof'ldnift. of the drift amplifier;

on automatic operation. The gate will then" drift the remaining distance into the signa1,.10ck.:n the signal pulse, and follow the signal pulse from that point. a

The circuit channel used to generate the auto-- matic gain control voltage to be fed back to the receiver I3 is-similanto the :initial. stages of the.

automatic tracking channel. This. automatic gairr contro1 voltage 'is-designed tovary with: the peak 'amplitud'e-ofthe pulse signa and to. exelude-spurious pulses.rand-random noises which exist-in thereceivingunit. Accordingly; the video signal output .of: the receiver I 3 isfede.to another.

gated voltmeter circuit '20 ,1. which: is gated.- by :the microsecond gate generator 0 I I whose' output surrounds the signal pulse in time. The output ofthe voltmeter 20-is fed through a direct cur rent filter-2| which. averages over the period of one:cycle of operation theamplitude of the -10-' microsecond gate and any signal pulses or' ran dom -noisepulses lwhiclt may occur at the I inter valofirecurren'ceof th'e lO micro-second gate. and 1 irr time coinci'de'nce- -therewith. This. average-- D'."+CIL.:voltage is then coupled through a D 0- amplifir- 22 where the :,;variations-of: the =D.-C'. voltage level in its output are amplified With-re spect'to the variation; of the output of the gated voltmeter- 20E" This varying-DEC.- voltage is then coupled to a secondamplifier 23 having short-1 tiine const'ant output andinput circuits so that the amplifier 23i acts asa quick acting:amplifierrather than-a drift amplifier and "as a conse quence producesrelatively: fast automatic gaincontrbl action. The output of this amplifier-231s fed "through aconventional bleed'en and direct current" filterarrangement to T the: gain control unit 24- 'of the receiver I 3 with which: the system is =associatedto contr olthe gairi'of signals in' the receiver;

Referringnow to Fig; 2; showing'the schematic diagram-ofthecircuit constituting the'automaticgairi controlsystem, the'three-microsecond gating puls'e-is-maintained in coincidenceW-ith the pulse signal in -the -receiver I3 by the introduction of variable delay 7 into the automatic tracking channel; controlledas a function of-thetimeoc--- currence-of the: signal pulsev with respect to: the occurrence of the cycle initiating-reference.pulse by means'of a phanta'stron' circuit asfollows.- Aninitiatingtpulse, obtained "from: any conventional external: source and controllingthe recurrent ire quency of the QCYCIGS of operation: of: thel'trans mittinge and" receivingsystem 4 with: which the Y auto'matic gain' control. system is: operating;: is carried-through a connecting lead IDI to the suppressor grid of tube I02, which is F-a' special pento'd'edesigned for phantastron operation;- Tliis initiating pulse triggers the-phantastron- --I 02 driving its plate suddenly to a less positive voltage: From-this:initial*positionthe plate drifts graduallyto-"avoltage less positive than 'saidlat 40 11 channel;

'4 ter voltage as is characteristic of the phantastron circuit, fairly linearly, according to the constants of the parameters in the plate and grid circuits from a level which is determined by th voltage 5 on the plate of diode I03 to a lower level determined by the tube constants, the plate of diode I 03 being couplediidirectlyg to: the plate of-phantastron I02. This voltage-in turn -is determined by the plate voltage on the plate of triode ampli- 10 her tube I04 to which the cathode of the diode I03 .is... connected. A somewhat steeper-sided pulse is obtained on the screen grid of the phantastron-tube I0 2.

This pulse is differentiated by condensers I05 and-I llfi coupled directly to the screen grid of the phantastron tube I02, so that the trailing edge of the phantastron output pulse triggers the tube in 1 the'multivibratorgate generators I0 and II, Fig. 1; the'pulses being-coupled to the tubes through the plate I01 and the grid I08 of tubes I09 and I I0, respectively: These-vmultivibrators, comnected as conventional:

pulses ioccurringiat the :platesiof tubes. III and I I0 areithen'zcoupledito the automatic-followings andthe automatic gain control-voltmeter.circuits-1 to perform: the time gating operation... The: vthree. microsecond gate, taken from the plate-of tube; III in. the firstirmentioned gategenerator is fed; through: resistor" I I3 ian'dvcondenser H4 .toe ther grid of triode H5:whichrformsthe-vachum tube" '35- voltmeter for the. automatic following channel.

The: ten. microsecond 'gatea-is. couplediromethe plate. of tube l I01. throughiresiston H0 and .ca-. pacitor. 'I I I. to the. grid 'oftriode I I8 zform'ingcthe gated :voltmeter for'xthe 1 automatic #gain control. Video'output ofi'the pulse receiver- Iii is coupled from. :the terminal I I 9;; .thezvideoinput J terminal* to: the automatic gain control unit; through condensers I20 and I20 to the plates-off the 'gated voltmeter tubes 1 I51 and -IIB, respectively: The plates of. the-vacuum. tube voltmeters I I5 and I I8 arecoupled totheinput gridsof am- I24andacondensersI25and I26; The diode-I21 '50'Lplaced. across resistor I23 servea merely as an-os cillationesuppressingdiode. The plate of ampli fier triode 121 is coupled through resistor I28 to the plate of the drift amplifier tube I04. In op-- eration, when the pulse signal and thegate do '55:? not coincide inl-triode II5, theplate of tube H5 is approximately'at ground level. Therefore,- the amplifier I2l is conducting heavily; placing the plate'of' the amplifier I2 I' at low voltage tending to cut offdrift amplifier I04. However, due

BOT-to the. feed back action of condenser 129,- coupled between the plate and: the grid-ofthis amplifier I04, the plate of drift amplifier I04 -drifts,1positive quite slowly.

( Thevoltage level ofthe plate 'of drift-ampli 6'53fier -I-04 sets the-Width, through'theidiode' I03,-of

thefiphantastron pulse generated. by the. plianta's tron- I02. When the threemicrosecond gate'an'd thesignal coincidein-tube I I5, the-plate of tube oftube I04 thus bringingits piate ne gative and narrowing the] pulse output of the phantastron tube I025 However, in so doing, the gate istaken away from the signaLjallowing the plate of the vacuum tube voltmeter. I I to drift toward ground again, so that the act njis reversed. The net result is the three microsecond gate will rest on theleading edge of the pulse signal. The diode I21 is used to givean asymmetrical time constant,

thereby minimizingthetendency of the gate to oscillate; Themariual gate control is inserted at'the inputfgrid of the tube I04. The automatic control'voltage. on the plate of the amplifier I2I is passed through a connector tofterminal I310 of a manual-automatic control switch i9 The grid of tube IMis'cOnne'ctedthrough a resistor I3I to the movable arm I32 of the control switch 13.

adjustment of the cut .offvoltages of tubes 5' tion of the D.-C. voltage level on the grids of the tubes .I I5 and II8 and on the cathodes of the tubes I2I1andI22, the threshold values oflpulse amplitude necessaryto operate the circuit may purposes without the payment of be determined. That is, the circuits may. be so adjusted as notto operate 'on a pulse smaller than some j predetermined amplitude, thereby preventing the automatic following circuit from. tracking noise and low voltage signals. Since the.

signalpulses picked up in the receiver I3 will recur each cycle of operation at substantially the same time with respect to the time of occurrence of an initiating pulse, or will only very smoothly and in small increments therefrom, while noise signals will occur in a random abruptly varying time relationwith the initiating pulse, the automatic following circuit will tend to lock on the recurring signal pulse, even when the amplitude of the random noise is greater.

.The. automaticgain control generator channel is quite. similar to the automatic follower. channeljust described. The automatic gain control channel is. composedotthe vacuum tube volts meter stage H8, a D.-C.amp1ifi"er I22,and ,a quick acting D.-C. amplifier. I35, .the grid of which is directly connected to the plate of the D.-C. amplifier I22. The ten microsecond gate of the gate generator composed of tubes I Ill and H2 is used to gate the vacuum tube voltmeter I I8 for the reason that the ten microsecond gate surrounds the pulse signal in time. When the signal and the gate coincide, a 11-0. voltage is developed at the plate of the voltmeter tube II8 which will be-proportional to the amplitude of the input signal pulse. This voltage is filtered bythe resistor I24 and condenser I26 and passed through'the amplifier I22 to the' quickactin g amplifier I where a rapid'r'esponse D.-C."voltage is generated which is proportional to the absolute amplitude of any signals occurring during the interal of the ten microsecond gate, thus the interval of conduction of voltmeter tube II8. This automatic gain control voltage is taken off at the plate of quick acting amplifier I35 and is fed through the potentiometer I36 and a conventional smoothing filter comprising condenser 6 I31 having the proper value of capacitance shunt to ground the ripple frequency variations present in the D.-C. control voltage fed to'the gain control unit 24 of the receiver I3 to control the gain of this unit. In'accordance with con ventional practice. this automatic gain control voltage will be coupled as avaryingD-C. volt-- age to thegrid of one or several of the stages in the intermediate frequency strip of the-"res! ceiver I3 to control the gain ofthese stages and" therefore the amplitude of the video output'of the receiver as a function of the absolute pulse amplitude fed thereto from the antenna.

Various modifications may be made in the in vention without departing from the spirit or scope thereof and it is desired, therefore, that only such limitations shall be placed thereon as are imposed by thep'riorart and areset forth in the appended claims. I

f The invention described herein may be man'u factored and used by'or for theGov'ernment of the United States of America for governmental thereon or therefor. v

Whatis claimed-is: I

1. An automatic gain control voltage gener ating system for a pulse transmitting receiving' system comprising a source of reference pulses having substantially the same pulse repetition rate as the pulses transmitted by said pulse transmitting system, variable duration control pulse generating means having an input trigger" terminal and a duration control terminal incor porated therein, said input trigger terminal being coupled to said reference pulse source and responsive to a pulse therefrom to generate a control pulse, a first gating pulse generating means coupled to said control pulse means and responsive to a pulse therefrom to'generate a gating pulse of predetermined duration as chronal withthe trailing edge of said control pulse, a coincidence circuit coupled to saidfirst gating pulse means and tothe pulse receiver of said pulse transmitting-rece1ving system, volta e generating means fedby the output of said coin cidence circuit and coupled to said duration conj-* trol terminal of saidvariable duration control pulse generating means for generating a voltage the magnitude of which varies in a firstgivenf direction upon non-coincidence of said gating pulse and receivedjpulse and varies inthe op-' posite directionupon coincidence of said gating pulse and received pulse, a secondgating pulse generating means connected to said variable duration control pulse means and responsive to a pulse therefrom to generate a gating pulse of sufficient duration to overlap said received pulse upon coincidence occurring in said coincidence circuit, means coupled to said pulse receiver and said second gating pulse means responsive to a gating pulse to permit the passage therethrough of a coincident received pulse,and averaging means coupled to said last named means fordef riving adirect current voltage proportional to the amplitude of said received pulse. v

2. An automatic signal pulse tracking system for usein a pulse transmitting receiving system comprising a source of reference pulses having substantially the same pulse repetition rate as the pulses transmitted by said pulse generating system, variable duration control pulse generating means having an input trigger terminal and a duration control terminal incorporated therein, said input trigger terminal being coupled to said reference pulse source and responsive to a any royalties assas in pnlss therefrcm to; generates.- control pulse-sets 1 ing pulseegeneratin ;Ymeans :coupled to: said come trol pulsaameans. and responsive to a pulse there?- fromze to generatea. gating; pulse of V p redetermined duration isochronal with the .trai1ing: edgea ob said control. pulse; a coincidence. circuit. coupledxto-xsaidgating =pl lseqmeans and to the pulserreceiverof' said, pulse transmitting receiving, system; volta e enerating means fed b the out-- puttofwsaid coincidence circuit and coupled to said; duration; control terminalof. said variable duration: control; pulse: generating means for: generating; avvoltage: the magnitude Of..WhiCh. varies ;--in;a' first given direction upon non-coincidenceof said gating pulse-and receivedpulsa andivariesin the opposite direction upon. coin.- eidence; of; saidgating pulse. and; received pulse.

7 3. An. automatic signal pulse: tracking system for use in a pulse transmitting receiving .system; comprising a..sourceof reference pulses, variable duration-control pulses generating means having; an,lnput1trig er terminal and: a duration control terminal, incorporated therein-,. said .ina put trigger terminal being coupledto said reference pulse source and responsive toarpulsetherefrointo generatea: control pulse, gating: pulse gflneratingmeans coupled. .to said control -pulsemeans. and responsive to.-a pulse therefrom to; generatea gating pulse isochronal with'the trailing-edge of said control pulse, a coincidence circoupled tosaid gating pulse-means and. to the pulsereceiver ofsaid pulse. transmittingreceiving system; voltage-generating means fed. by the output or. said coincidence. circuit and. coupledtosaid duration. control terminal oi said variable duration control pulse generating. means for-generating avoltagethe magnitude of which varies..in,a, first given direction. .upon, non-coin.- eidence of. said gating pulse and ..received pulsev and varies in'the opposite direction upon coincidenceoi said gating pulse and, received. pulse.

4,.. In.a pulse. transmitting-receiving system the combination of an-automatic gain control channel for. controlling the. gain of. the pulse receiver inv said system. and an automatic signal pulse tracking channel. for fixing theposition'. ofgatingpulses relative to received signalpulses. said pulse tracking .channelv comprising. a, source ofreference pulseshaving substantially the same pulserepetition rate. as the pulses transmitted. by said pulsetransmitting system, voltage generating, means for. generatinga voltage. the magnitude of: which normally slowly varies in .a fir t. i

'sense, .,control pulse generating. means coupled.

toisaid voltag generating. means andsaidsource. of. reference pulses. responsive. to a. reference.

a s zl t a eee P12Q i-flmeem t mside =0 in enti i aa ceivedpulsaand.saidigaincontrol.channetcolnaprising. a second.gating-pulse.generatinaemeapsi coupled. to e-said gtracking channei.control pulsei generating means. responsive. tosa. pulse. vthfere fromto generate a; second. gating l pulse -wider. than. said first. pulseandof sumcientduration; to. .overlap .said :receivedpulse upon coincidence: occurring in said tracking channelcoincidencea. circuits. meanscoupled to said. pulsereceiver re:

sponsive. to a. gating pulse. Irom .sai'd;=.galn. con:

trolchannel .gatingpulse. means. to. .permit..the.. passage T therethrough of. a....coincident received;

pulseand averaging means. coupled to saidlast;

named meansior. deriving apirect.currentvolte age Proportional. to the. amplitude ot. ;sald.-.re=- ceived.pu1se,-.

5.; In; a. pulse transmitting receivmg system; the combination or. an, automatic: gain. control; channelior controlling. the gainnof; the.,.pulsez; receiver insaid. system. andan. automatic signal; pulse tracking. channel. for. fixin the 1 .95 1 0 of. gating pulses relative toreceived. signal .pulses,.. said .pulse tracking. channel comprising. a. source:

' of. referencepulses having substantially the same.

ncl; control pulse means responsive. toa. pulse:

pulseto. generate a, control pulse having adura:

tion. dependent on, the magnitude .of the voltage generatedby. saidv voltage. generating. means. first. gating pulse. generating means coupledto fsaidcontrolipulse means responsive to. a pulse. 6m

therefrom tov generate .a first... gating pulse of. predetermined duration. isochronal with the. trailing. edge of said. controlpulse, a..coincidence. circuit coupledgto saidgating pulse means and to. said pulse. receiver, and .means coupling said Qincidencecircuit tov said voltage generating. means for causing th'e voltage magnitude thereof pulse repetitionuratef as the pulses. transmitted f. bysaid pulse transmitting. system amusemtion. control pulse generating means coupled ;to. said source 0tv reference pulses responsive to. successive. reference pulsesto enerate a control. pulse.v for each. or, said. reference pulses. having. aleading, edge substantially in. isochronismwith a corresponding referencepulsa first gatingpulse. generating. means coupled to said. control pulse:- meansandresponsivetothe.trailingedge of 1;a.: pulse therefromto generate .agatingpulselmeans forinjecting, said .receivedInu-lses into said rack:

ing, channel). and. means operative upon. coin:

cidence. of... corresponding; gating, and. received" pulsesto. vary theduration ofithe control pulses ina firstsenseand upon non-coincidenceto vary; said duration intne. opposite senseand said gain: controLchannel.comprising;a second gatingipulsegenerating means coupledto ,said tracking. chan-,

therefrom... to. generate. a second gating; pulse wider than said first pulse and of, sufficient duragtionto. overlap. the received fpuls'e corresponding" toisaid controlpulsa. means coupledto said pulse receiver andsaid second gatingpulse meansresponsiveto asatin p lseto permit the. pas a e therethroughoila coincident received pulse. aild" avera ing. means. coupled to said last; named means. for; deriving a direct current volta e propprtionaltc. the. amplitude; of said received;

pulse..-

7 ROBERTA... JR:

REEERENCESf'CITED" The. following; references ,are of record m ths... file of this; patent a 1 UNITED STATESIPATENTSI. 

